In bipolar and bipolar complementary-metal-oxide-semiconductor (BiCMOS) circuit design, bandgap circuits are the method of choice to develop a stable voltage reference (e.g., voltage regulators incorporate a bandgap voltage reference) and to provide a bias current that is proportional to absolute temperature (PTAT). Both voltage regulators and PTAT current sources are widely used to bias analog circuits. Because the bandgap is fundamental to the biasing of any BiCMOS/Bipolar circuit, it is imperative that the bandgap circuit operate reliably. Failure of a bandgap circuit (e.g., a non-starter) can cause catastrophic failure of the integrated circuit.
Like other self-biasing circuits, bandgap circuits have two stable states: (1) normal operation (e.g., as designed), and (2) zero current, otherwise known as a "non-starter." Under normal operation, the bandgap circuit biases the rest of the integrated circuit ("IC") properly. In the non-starter state, however, the current is zero in each of the branches of the bandgap circuit. The bandgap circuit output (Vbg), which should be approximately 1.2 volts (the bandgap of silicon), remains close to zero (approximately 0.4 volts). In a non-starter bandgap circuit, the voltage regulators and PTAT current mirrors that depend on an accurate bandgap voltage are non-functional. Because an accurate bandgap voltage is necessary to the biasing of every BiCMOS/Bipolar circuit, a non-starter bandgap circuit renders the entire IC useless.
Several methods of starting a bandgap circuit, which is in a "non-start" mode either at initial start-up or due to a glitch such as a sudden interruption of power or a surge of power, are known in the art. In one of these methods, the power supply is snapped on quickly such that parasitic capacitance to ground at the drain of a p-type metal-oxide semiconductor ("PMOS") device in a current mirror holds that voltage low for a long enough period of time to start the current mirror, thereby starting the bandgap circuit. This method does not work if the power is brought up gradually from zero volts to Vcc.
Another method for starting bandgap circuits known in the art is to rely on the inherent leakage currents that exist in a PMOS transistor to maintain current mirror operation after a glitch. This method suffers from two major drawbacks. First, the recovery time after the glitch is slow due to the low levels of leakage current (i.e., on the order of microseconds). Second, the leakage currents which this method relies upon are highly dependant on the fabrication process and operating temperature, causing serious start-up reliability concerns.
Yet another method for starting bandgap circuits is to place a large value resistor from a critical node in the bandgap circuit to ground. This method suffers from three major problems. First, the intentional leakage path created by the resistor draws additional current, even when the bandgap circuit is operational. Second, the current must be leaked equally with respect to current mirrors to prevent an imbalance in biasing. Third, the very large value resistor required to implement this method is difficult to realize monolithically.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following United States patents and technical literature.
U.S. Pat. No. 5,453,679 (issued to Rapp) shows a bandgap constant voltage circuit with a start-up circuit using a P-channel transistor and an N-channel transistor to form an inverter which switches on another N-channel transistor, causing the latter N-channel transistor to pull current through a P-channel transistor in a current mirror in the bandgap circuit when an Nref node in the bandgap circuit is low due to a non-start condition. The current pulled through the P-channel transistor starts the bandgap circuit. Although this patent provides hysterysis in the start-up circuit to mimimize oscillation, it does so by delaying transition at the inverter output. The start-up circuit disclosed in this patent does not provide sharp transitions (i.e., short switching time) and, therefore, requires a long period of time to start a bandgap circuit. This start-up circuit does not disclose or claim transition voltages and the difference between them. Also, the start-up circuit of the '679 patent requires eight transistors, a requirement that can adversely affect cost and reliability.
U.S. Pat. No. 5,852,376 (issued to Kraus) shows a power-on detect circuit. The circuit uses an inverter to suppress a bandgap signal to a differential amplifier until the bandgap reference voltage is stable.
U.S. Pat. No. 5,747,978 (issued to Gariboldi et al.) teaches a circuit for generating a reference voltage and detecting an under-voltage using a voltage divider (which can comprise resistors in series). The voltage divider provides an input to a comparator (which can comprise a bandgap circuit). The circuit further comprises a feedback network connected between the output of the comparator and a second input. For low supply voltages, the feedback loop is open and the voltage divider provides a voltage proportional to the supply voltage at the comparator input. As the voltage exceeds the threshold voltage (bandgap), the feedback loop holds the comparator input voltage at bandgap voltage. This circuit will always draw current, and therefore consume power when the bandgap circuit is operational. In addition, the '978 patent does not disclose or claim recovery time following a glitch.
U.S. Pat. No. 5,367,249 (issued to Honnigford) shows start-up circuitry for a bandgap reference circuit for initial power-up. The circuitry uses a power divider formed by two transistors to switch on a transistor coupled to a branch of the bandgap circuit, pulling current and starting the bandgap circuit. The start-up circuit disclosed in the '249 patent does not have a feedback loop providing hysterysis and sharp transitions.
A. Paul Brokaw, "A Temperature Sensor With Single Resistor Set-Point Programming," IEEE Journal of Solid-State Circuits, Vol. 31, No. 12, pages 1908, 1915 (December 1996), teaches a set-point temperature switching circuit using an hysterytic feedback loop with bipolar transistors.
The deficiencies of the conventional circuitry and methods for starting a bandgap circuit show that a need still exists for improvement. To overcome the shortcomings of the conventional circuitry, a new circuitry and method for starting a bandgap circuit is provided. It is an object of the present invention to provide circuitry and a method for starting a bandgap circuit during initial power-up or following a power glitch. It is another object of the present invention to provide an hysterytic circuit and a method for starting a bandgap circuit such that oscillation about the inverter transition voltage is prevented. It is a further object of the present invention to provide a circuit and method for starting a bandgap circuit which will restore bandgap operation in less than one microsecond using an inverter with a feedback loop to accelerate switching.